Hydrogen bonding in the crystal structure of phurcalite, Ca(2)[(UO(2))(3)O(2)(PO(4))(2)]·7H(2)O: single-crystal X-ray study and TORQUE calculations

The crystal structure of phurcalite, Ca(2)[(UO(2))(3)O(2)(PO(4))(2)]·7H(2)O, orthorhombic, a = 17.3785 (9) Å, b = 15.9864 (8) Å, c = 13.5477 (10) Å, V = 3763.8 (4) Å(3), space group Pbca, Z = 8 has been refined from single-crystal XRD data to R = 0.042 for 3182 unique [I > 3σ(I)] reflections and the hydrogen-bonding scheme has been refined by theoretical calculations based on the TORQUE method. The phurcalite structure is layered, with uranyl phosphate sheets of the phosphuranylite topology which are linked by extensive hydrogen bonds across the interlayer occupied by Ca(2+) cations and H(2)O groups. In contrast to previous studies the approach here reveals five transformer H(2)O groups (compared to three expected by a previous study) and two non-transformer H(2)O groups. One of the transformer H(2)O groups is, nevertheless, not linked to any metal cation, which is a less frequent type of H(2)O bonding in solid state compounds and minerals. The structural formula of phurcalite has been therefore redefined as {Ca(2)(H(2) ([3])O)(5)(H(2) ([4])O)(2)}[(UO(2))(3)O(2)(PO(4))(2)], Z = 8.